ci(release): disable tauri-action updater repack, upload .sig ourselves

Root cause: tauri-action runs a second "Packaging" pass after tauri CLI
finishes, which rewrites src-tauri/target/<target>/release/bundle/macos/
Psysonic.app.tar.gz with different bytes. The .sig produced by tauri CLI
no longer matches the new hash, so tauri-action silently deletes it —
leaving the directory with only .app and .app.tar.gz (no .sig for our
manifest generator to consume).

Fix: pass `includeUpdaterJson: false` to tauri-action so it skips the
repack + updater JSON upload entirely, then copy both the .app.tar.gz
and .app.tar.gz.sig produced by tauri CLI into the workspace root with
the expected asset names and `gh release upload` them.

Also disables build-linux and the Windows matrix entry during testing
to cut iteration time roughly in half.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src-tauri/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "psysonic"
-version = "1.34.17"
+version = "1.34.18"
 description = "Psysonic Desktop Music Player"
 authors = []
 license = ""